1. Field of the Invention
The present invention relates to an adhesive resin composition for a laminate having improved properties such as gas barrier properties (against oxygen, carbon dioxide gas and so on), mechanical strength, barrier properties against contents involved therein (e.g. flavor and odor protection) and appearance (e.g. surface gloss and transparency) which are widely required for various industrial applications such as food packaging materials.
More specifically, the present invention relates to adhesive resin compositions which can be widely used in various fields for lamination of materials independently selected from the group consisting of gas barrier resins, metals such as steel plate and aluminum foil, etc, polyolefins (hereinafter, sometimes abbreviated as "PO") such as polypropylene, polyethylene and 4-methyl-1-pentene resin; styrenic resins (hereinafter, sometimes abbreviated as "PS") such as general-purpose (GP) polystyrene, high-impact (HI) polystyrene, styrene/methacrylic acid copolymers, acrylonitrile/styrene resins, acrylonitrile/butadiene/styrene resins and modified acrylonitrile/butadiene/styrene resins; polyester resins (hereinafter, sometimes abbreviated as "PES") such as poly(ethylene terephthalate) (PET), PET copolymer, poly(butylene terephthalate) (PBT), poly(ethylene naphthalate) (PEN) and poly(cyclohexylene terephthalate) (PCT); polycarbonate (hereinafter, sometimes abbreviated as "PC"); acrylic resins (hereinafter, sometimes abbreviated as "PAN") such as polyacrylonitriles, acrylonitrile/methyl acrylate/butadiene copolymers and poly(methyl methacrylate); saponified ethylene/vinyl acetate copolymers (EVOH) having ethylene content of 15 to 65 mole% and saponification value of not less than 90% or their mixtures with at least one kind selected from polyamide, polyester, polycarbonate and platelet fillers (hereinafter, sometimes abbreviated as "EVOH resins, etc."); polyamide (hereinafter, sometimes abbreviated as "PA") such as nylon 6, nylon 66, nylon 6-66, nylon 12 and polyamide containing xylylene group; and so on.
2. Related Art
In various fields, polyolefins such as polypropylene and polyethylene and styrene-based resins such as HI polystyrene and GP polystyrene have been widely used. Polyolefins have excellent moldability, mechanical strength and chemical resistance, but are not so good in gas barrier properties and barrier properties against contents of the finished articles and may be impaired in appearance (e.g. surface gloss and transparency) depending on the methods for molding or the kinds of resins employed. On the other hand, styrene-bases resins have excellent moldability and stiffness, but are not so good in gas barrier properties and oil resistance.
For the improvement of these defects, various techniques have been proposed, such as lamination with EVOH or polyamide which have excellent gas barrier properties. In addition, polyolefins are recommended for lamination with polyester or polycarbonate in order to improve barrier properties to contents and appearance. Polystyrene are recommended for lamination with polyolefins having excellent oil resistance, such as polypropylene and polyethylene. Acrylonitrile/styrene resins and acrylonitrile/butadiene/styrene resins are also recommended for lamination with metals or polyolefins.
However, since polyolefins or polystyrenes have no polar group, if they are directly laminated with EVOH or a polyamide, the interfacial adhesion is extremely weak. These polymers also exhibit similar weak interlayer adhesion to other resins. Therefore, these polymers are not practically available as lamination materials.
For improving such defect, the use of various adhesives have been proposed. For example, for the bonding of PO with polar resins such as EVOH and PA, polyolefin adhesive resin modified with unsaturated carboxylic acid and its derivative are generally used as the adhesives, which has no problem in practical application.
On the other hand, as for the laminates of PO with resins other than EVOH and PA, that is, the laminates of PO with PES, PC, PAN, PS and so on, various adhesives have been proposed such as a composition of an olefinic polymer and an alicyclic or aromatic polymer (see Japanese Patent Laid-open No. 50-116536), a composition of an ethylene/vinyl acetate copolymer (hereinalter, abbreviated as "EVA") and a tackifier (see Japanese Patent Laid-open Nos. 53-147733 and 54-10384), a composition of an EVA and an aliphatic petroleum resin as a tackifier (see Japanese Patent Laid-open No. 53-127546), a composition of a modified crystalline ethylene/.alpha.-olefin random copolymer and a tackifier (see Japanese Patent Laid-open No. 61-241144), a composition of a low crystalline (low-density) ethylene/.alpha.-olefin random copolymer, a tackifier and a modified polyethylene (see Japanese Patent Laid-open No. 61-162539), a hot melt adhesive composition comprising a mixture of a styrenic thermoplastic elastomer, a tackifier, a low molecular weight polypropylene and a process oil (see Japanese Patent Laid-open No. 64-144483), a hot melt adhesive composition comprising a SBS, a SEBS, an alicyclic tackifier and a cyclo-olefin random copolymer (see Japanese Patent laid-open No. 3-223381), and so on.
In any of the methods described above, however, a composition of an elastomer having a low density (i.e. low crystallinity), a waxy low molecular weight resin or an EVA and a tackifier is used as an adhesives. Therefore, the laminate of a PO resin with a PES, a PC, a PAN or a PS, or the laminate of a PS resin with an EVOH resin, a PA resin, a PES resin or a PC shows insufficient adhesive strength at ordinary temperatures (around 23.degree. C.), as well as at medium temperatures (around 60.degree. C.) where the postforming of the molded article (e.g. punching of the article after vacuum forming, flash cutting of the article after blow molding, etc.) is carried out and at the high temperatures (around 90.degree. C.) required for the boil sterilization of the molded article; which is still remains a problem for practical application at the present time.